Hirsuties and female androgenetic alopecia are common androgen-dependent problems. For example, each year about 200 women with these conditions are referred to the Department of Dermatology in Sheffield; patients are also referred to endocrinologists and gynaecologists. Hair is an important cosmetic asset and although these conditions are not physically disabling they can have a profound adverse affect on the quality of life. Current treatments for hirsuties are, at best, only modestly effective. There is no worthwhile treatment for female androgenetic alopecia and, in most cases, management is limited to ensuring that there is no serious endocrine pathology and prescribing a wig for severely affected patients. Better treatments are needed but their development is hampered by our poor understanding of the basic biology. In particular, we do not understand why androgens stimulate hair growth over most of the skin but inhibit hair growth on the scalp.
Balding is a common alteration in the pattern of scalp hair growth. It affects both men and women although the degree of hair loss is less and tends to occur in a more diffuse pattern in women. Genetic factors are thought to be important and there are racial differences in the prevalence. There is strong evidence that androgen hormones are necessary for the development of balding but the mode of hormone action on the hair follicle is unknown. At present there is no satisfactory treatment. However, the following have some proven benefit;
Minoxidil
Topical application of minoxidil produces modest recovery in early balding in about 30% of subjects. Minoxidil is a potassium channel agonist in vascular smooth muscle. A number of other potassium channel agonists have been reported to stimulate hair growth (e.g diazoxide, nicorandil) suggesting that this pharamacological property is relevant to their hypertrichotic effect. However the mechanism of action is unknown. PA1 This is a steroidal androgen receptor blocker which also has progestational activity. Several small trials indicate a small positive affect in female hair loss. CPA has to be taken orally and therefore cannot be used in men (whom it would feminize). There are no topical formulations available as far as we know. PA1 These techniques are not readily available and have very limited applications. Numerous other products have been promoted for the treatment of balding but, other than those listed above, there is no adequate clinical data on the results. PA1 In all mammalian species to have been studied (various rodents, dogs), estrogens inhibit hair growth (8). PA1 Aromatase activity has been detected in plucked human scalp hair follicles (9). PA1 Aromatase protein has been detected in human scalp follicles by immunochemistry (10). PA1 We have recently found evidence of aromatase activity in vivo in rat hair follicles. This was an incidental finding in a study designed to measure the effect of a novel pre-estrogenic steroid on sebaceous secretion. The compound in question is metabolized to estrogen by the action of aromatase and will only act as an estrogen in tissues which possess estrogen receptors and aromatase activity. Topical application inhibited hair growth in the rat by about 35%. PA1 In genital skin fibroblasts, aromatase activity is stimulated by testosterone via an androgen receptor mediated mechanism (11). PA1 In hair follicles in most body sites, testosterone acts as a pre-hormone and the response to testosterone is determined by which metabolic pathway predominates. In skin sites where 5.alpha.-reductase activity is high, such as in the beard, testosterone is metabolised to DHT which has a stimulatory action on hair growth. In scalp, testosterone is metabolised mainly via the aromatase pathway to estrogens causing inhibition of hair growth. Aromatase activity is under receptor-mediated androgenic control; hence balding does not occur in testicular feminisation. The greater severity of balding in men compared with women can be explained by higher levels of oestrogen being attained in the male scalp by virtue of androgen stimulated aromatase activity. An analogous situation exists in the brain where certain aspects of male behaviours in a number of avian and mammalian species are determined by oestrogens synthesised locally by androgen-dependent aromatisation of androgens (12). This hypothesis is also able to encompass a role for 5.alpha.-reductase; as DHT is a more potent androgen than testosterone, circulating or locally formed DHT can be a more effective inducer of aromatase. PA1 There will be a higher level of aromatase activity in skin sites prone to balding i.e frontal scalp and vertex than in non-balding sites such as the occipital scalp and the beard. PA1 Skin aromatase activity will be induced by androgens via the androgen receptor. PA1 Skin fibroblast lines from beard (N=5,) occipital scalp (N=5) and frontal scalp (N=7) all possessed aromatase activity. Enzymic kinetic values were established by incubating cells with the substrate (androstenedione) in a range of concentrations. The highest activities were found in cell lines from the frontal scalp i.e from a potentially balding site. Mean V.sub.max (fmol/mg protein/hour)+/- SEM in beard, occipital and frontal cells were 892+/-155, 914+/-188 and 2,817+/-866 respectively. There was no difference in the Km for androstenedione (mean=25 nmol) between the three sites. This value is similar to that quoted for other tissues. PA1 Fibroblast aromatase was stimulated by pre-incubating cells with testosterone at physiological concentrations (10 nmol). 19 cell lines were studied. Mean fold increase in aromatase activity (+/- SEM) was 2.69+/-0.38 (p&lt;0.02 paired test). In separate experiments on three cell lines the response to a range of concentrations of testosterone and DHT was studied. Testosterone gave a twofold increase in aromatase maximal at 10 nmol. At the same concentration in response to DHT was over twice as great (4.3 fold). This response to testosterone DHT was inhibited by cyproterone acetate (which blocks androgen receptors) indicating that androgen stimulation of skin aromatase is via an androgen receptor dependent mechanism. PA1 No aromatase was detected in hair follicle dermal papilla cells cultured through any of the above sites (fourteen cell lines). Testosterone failed to induce aromatase in dermal papilla cells. PA1 Dexamethasone stimulated aromatase 10-20 fold in fibroblasts (six cell lines) and induced aromatase in dermal papilla cells (four cell lines). PA1 Fibroblast aromatase was inhibited by 4-OH androstenedione, a known inhibitor of aromatase in other tissues. Kinetic studies in three cell lines were consistent with irreversible competitive inhibition. 50% inhibition occurred at approximately 25 nmol concentration.
Cyproterone Acetate (CPA)
Hair Surgery eg. hair transplantation, scalp reduction.
Development of new effective treatments for balding has been limited by our poor understanding of the basic biology, and the absence of suitable animal models for the routine testing of compounds.
Biological Background
Androgen hormones (e.g testosterone) are the major systemic influence on hair growth in man. In most body sites, such as the beard and pubic skin, androgens stimulate hair growth by prolonging the growth phase of the hair cycle (anagen) and increasing follicle size. Hair growth on the scalp does not require androgens but, paradoxically, androgens are necessary for balding on the scalp in genetically predisposed individuals (androgenetic alopecia) where there is a progressive decline in the duration of anagen and in hair follicle size. Men castrated before puberty fail to grow beards and do not go bald. If subsequently treated with testosterone about one third of male castrates will show balding (1). Androgenetic alopecia is also common in women where it usually present as a diffuse hair loss rather than showing the patterning seen in men.
Current thinking is that testosterone is necessary for the development of balding but as only a proportion of men show balding, and there is no difference in circulating testosterone levels between bald and non-bald men, other factors must be needed for hair loss to occur. Until this work was carried out there was little indication as to what these other factors might be. Further evidence implicating androgens in balding (and growth of beard and body hair) is that genetic males who lack intracellular androgen receptors [testicular feminization] fail to grow body hair and do not go bald in spite of high circulating levels of testosterone. It is generally believed that in most body sites, testosterone is metabolised in the skin to the potent androgen dihydrotestosterone (DHT) by the enzyme 5.alpha.-reductase before acting on hair growth. There is considerable literature relating to the involvement of DHT in hair loss in both males and females (WO-A-8 502 543, WO-A-8 602 269, EP-A-0 566 979, EP-A-0 163 490, WO-A-8 601 402, DE-A-2 840 144, U.S. Pat. No. 4,684,635, J. Steroid Biochem. Vol. 19, no. 1, pp. 587-590, 1983, JCE and M, Vol. 39, no. 6, pp. 1012-1019, 1974, JCE and M, Vol. 38, no. 5, pp. 811-819, 1974 J. Endocr., Vol. 59, no. 2, pp. 345-351, 1973 J. Cutan Pathol 1991: 19: 309-314 and M. E. Sawaya: Steroid Chemistry and Hormone Control during the Hair Follicle Cycle, pages 377 to 384). Genetic males with type 2 5.alpha.-reductase deficiency have normal or raised circulating testosterone levels but show a female pattern of body hair and sparse or absent beard growth. It is said that men with type 2 5.alpha.-reductase deficiency do not go bald but the evidence implicating this metabolic pathway in balding is less clear cut. Nothing is known of the prevalence of balding in the normal populations in which these cases have been reported and studies to date have failed to demonstrate the type 2 form of 5.alpha.-reductase in normal scalp. Administration of 5.alpha.-reductase inhibitors in macaques, a primate species which undergoes androgen-dependent scalp hair loss, has been reported to prevent and partially reverse the balding process but there is no clinical trial data published in humans. If DHT is involved in balding it would appear that the effect is derived from circulating DHT, not DHT synthesised in the scalp. This fails to explain why only a proportion of men show balding characteristics.
Hair is a keratinized differentiation product of a highly proliferative population of epithelial cells situated at the base of the hair follicle (the hair bulb matrix). Hair is not produced continuously; individual hair follicles undergo a repetitive sequence of growth (anagen) and rest (telogen) known as the hair cycle. Control of the hair cycle is determined primarily within the hair follicle itself but this intrinsic behaviour can be modulated by systemic influences mainly mediated by the endocrine system (2). The hair follicle also contains a dermal component derived from embryonic mesenchyme. This comprises the dermal sheath surrounding the follicle and the dermal papilla which invaginates the base of the hair bulb. Transplantation studies in rodents have shown the dermal papilla plays a vital role in hair growth. It is responsible for inducing growth and differentiation of follicular epithelium. It determines the follicle type (whisker, pelage etc.) and probably the diameter of the hair and there is indirect evidence that the dermal papilla is responsible for regulating the hair cycle (3,4). A number of studies have suggested that, in follicles where androgens stimulate hair growth, the hormone acts primarily on the dermal papilla. Dermal papilla cells cultured from human hair follicles express saturable, high affinity androgen receptors and the receptor concentration is higher in cells from beard follicles than from occipital scalp follicles (a relatively non-androgen responsive site) (5). Androgen receptors have also been demonstrated in the nuclei of dermal papilla cells in tissue sections by immunohistochemistry using a specific monoclonal antibody (6). Significantly, there was no antibody staining of cells in the epithelial compartment of the hair bulb. Dermal papilla cells cultured from beard follicles also express type 2 5.alpha.-reductase whereas those from scalp follicles do not (7). Type 1 5.alpha.-reductase is present in the scalp but men with 5.alpha.-reductase deficiency have normal levels of this enzyme. Therefore it seems likely that androgen action on the beard is mediated by DHT acting primarily on the dermal papilla but it is difficult to explain the inhibitory affect of androgens on scalp hair growth on the same basis.
In conclusion, the prior art is confusing in that it fails to explain the observed pattern of hair loss in a proportion of the population. It also provides no reasonable hypothesis on which to either predict a pre-disposition to hair loss and little basis upon which to base a systematic search for new and improved treatments.
Our investigations have instead lead us to study in some detail aromatase, a cytochrome p450 enzyme which catalyses the metabolism of androstenedione and testosterone to estrone and oestradiol respectively. It is localized mainly in the ovary and the placenta and participates in the regulation of reproductive functions. Aromatase activity is also found in extra-gonadal tissues such brain, liver adipose tissue and genital skin fibroblasts. The aromatase pathway of androgen metabolism has received scant interest in hair biology and little is known about the effect of estrogens on human hair growth. However a number of lines of evidence suggested to us that it may be important:
As a result we formulated the following hypothesis: